Viscous liquids stored in tanks need to be removed to be processed. Due to high viscosity the removal of highly viscous liquid, such as crude oils, fuel oils, lubes oils, asphalt, paint, syrups, present technical problems. Viscosity of liquids substantially depends on their temperature. All viscous liquids will solidify if cooled to corresponding low temperatures called a solid point. To prevent solidification of the viscous liquid in the storage tanks, the liquid in the tank must be heated to retain the viscous liquid in the liquid state (pour point). The pour point is an indication of the lowest temperature at which a viscous liquid can be stored and still be capable of flowing under very low forces. The absolute values of the solid point and the pour point depend on the nature of the viscous liquids and their compositions. The pour point temperature is about 5.degree. F. above the solid point temperature. Usually the storage tanks have heating coils for heating the liquid to keep it movable. At this temperature the viscosity is too high to allow liquid pumping from the storage tank. The further reduction of viscosity tis accomplished by use of an additional heat exchanger.
Heavy fuel oils are byproducts of regular refining operation. The naphtheneoes, aromatics and unsaturated hydrocarbons constitute the bulk of the heavy fuel oil. The amount of wax is relatively small. Depending on the source the pour point of the No. 6 fuel oil may be in the range from 5.degree. F. to 80.degree. F. For waxing No. 6 fuel, the pour point can reach 115.degree. F. For commercial purposes the residual fuel oils are mixed with light fractions of hydrocarbons in order to satisfy the ASTM standard specification for fuel oils.
Viscosity is a single most important property of the fuel oil because of difficulties in the handling and atomizing of the oil. The viscosity of the fuel oil at the pour point is too high to be pumped out of the storage tank. Usually the storage temperature is 20-40.degree. F. higher than the pour point. The high storage temperature of fuel oil results in increased heat losses from the storage tank, and may effect the oil stability.
The U.S. Department of Commerce in its standard CS12-48 recommends maximum practical limit viscosity for pumping 4000 Saybolt Universal Seconds (70-120.degree. F. range). For this purpose the fuel oil is heated by a heat exchanger immersed in the storage tank. Since the heater is installed on the suction line of the fuel pump, they are known as fuel tank suction heaters.
The commercial No. 6 fuel oil is heated in the fuel tank suction heater to the temperature 120-130.degree. F. The further heating is restricted to prevent flash points (about 150.degree. F. for No. 6 fuel oil). The flash point indicates the temperature below which a fuel oil can be handled without danger of fire. In order to prevent the evaporation of the fuel oil during the heating and, pump cavitation, suction heaters are designed for low pressure drop (maximum 2.5 psi). The maximum suction at the pump should be restricted to 18 inch of Hg to allow for pump wear and capacity variations with suction loads. The maximum lift from the bottom of the oil suction pipe to the pump inlet should not exceed 12 feet.
The fuel oil pump and heater system must be adequate to heat the heaviest oil that is to be burned at the maximum rate of consumption in the coldest weather. The pumps should deliver the maximum quantity under pressure specified for the type of burner used. When the steam consumption is reduced, the burner requires less fuel oil. The excess is discharged through the spring-loaded bypass valve to the pump suction or the storage tank. Tank suction heaters raise the fuel oil temperature sufficiently to reduced the viscosity, thus facilitating straining and pumping. The heater located on the discharge side of the pump heats the fuel oil to lower the viscosity of the fuel oil, so that it can be atomized effectively by the burners. Usually burner manufacturers specify the temperature, which is best suited for their burners. For instance, No. 6 fuel oil is heated to 160.degree. F. in horizontal rotary burners, to 220.degree. F. in steam atomizing burners, and to 250.degree. F. in mechanical atomizing burners.
In summary, the modern system of storage and discharge fuel oil requires oil heating in the storage tank by heating coils located on the bottom of the tank, a suction tank heater, installed on the suction side of the pump and an additional heater, installed on the discharge side of the pump. This commonly used method has a number of disadvantages.
First of all, the heating media (steam, hot water, or high temperature liquids) usually have higher pressure than the fuel oil pressure in the tank. In the tank when the suction heater is leaking, the heating media will penetrate the storage tank and contaminate the fuel oil. It is also possible to contaminate the boiler water if the pressure in the storage tank is higher than the pressure in the oil supply line. The repair or removal of the tank suction heater presents technical problems when the storage tank is filled with fuel oil. Secondly, the temperature of the heating media is restricted to prevent the evaporation of the fuel oil in the tank suction heater working and under vacuum. The low temperature of the heated media and the low permitted pressure drop results in a larger size of the tank suction heater. Finally, the maximum lift from the underground tanks is restricted due to pressure drop of the suction heater.